Water transport by the Na+/glucose cotransporter under isotonic conditions.
Solute cotransport in the Na+/glucose cotransporter is directly coupled to significant water fluxes. The water fluxes are energized by the downhill fluxes of the other substrates by a mechanism within the protein itself. In the present paper we investigate the Na+/glucose cotransporter expressed in Xenopus oocytes. We present a method which allows short-term exposures to sugar under voltage clamp conditions. We demonstrate that water is cotransported with the solutes despite no osmotic differences between the external and intracellular solutions. There is a fixed ratio of 195:1 between the number of water molecules and the number of Na+ ions transported, equivalent to 390 water molecules per glucose molecule. Unstirred layer effects are ruled out on the basis of experiments on native oocytes incubated with the ionophores gramicidin D or nystatin.
Zeuthen
T
T
Panum Institute, University of Copenhagen, Department of Medical Physiology.
Meinild
A K
AK
Klaerke
D A
DA
Loo
D D
DD
Wright
E M
EM
Belhage
B
B
Litman
T
T
D000818:Animals; D000900:Anti-Bacterial Agents / Q000494:pharmacology; D001692:Biological Transport / Q000187:drug effects / Q000502:physiology; D004594:Electrophysiology; D005260:Female; D005947:Glucose / Q000378:metabolism; D006096:Gramicidin / Q000494:pharmacology; D007476:Ionophores / Q000494:pharmacology; D007552:Isotonic Solutions; D008562:Membrane Glycoproteins / Q000378:metabolism*; D009004:Monosaccharide Transport Proteins / Q000378:metabolism*; D009761:Nystatin / Q000494:pharmacology; D009865:Oocytes / Q000737:chemistry / Q000502:physiology; D012964:Sodium / Q000378:metabolism; D051269:Sodium-Glucose Transporter 1; D014867:Water / Q000378:metabolism*; D014982:Xenopus laevis
D000900:Anti-Bacterial Agents; D007476:Ionophores; D007552:Isotonic Solutions; D008562:Membrane Glycoproteins; D009004:Monosaccharide Transport Proteins; D051269:Sodium-Glucose Transporter 1; D014867:Water; D009761:Nystatin; D006096:Gramicidin; D012964:Sodium; D005947:Glucose